1. Field of the Invention
The present invention relates to remotely controlled actuators for opening, closing and monitoring status of electrical power distribution switches of the type commonly used on overhead pole installations which close the contacts to carry high current--voltage and open to break the circuit with an ambient air gap separating the contacts.
2. Background Art
The most predominant method for routing electrical power to utility customers is through the use of single or three-phase electrical circuits carried on overhead power distribution lines. To allow these circuits to be disconnected, rerouted or otherwise reconfigured, air-break, single or gang-operated three-phase switches are commonly employed. These switches are typically mounted at the top of a wood, steel or concrete pole, and the operating actuator is typically carried down the pole on a wood, steel or fiberglass rod or shaft. The method for operating these switches is primarily manual, with a human operator or "lineman" unlocking a security padlock and operating a handle or shaft to close/open the switch. Both rotating shafts, requiring torsional operation, and reciprocating shafts, requiring and up and down operation of the shaft are commonly in use.
Within the last several decades, as automation and remote operation of the power system have become more economically feasible, motor-driven systems (motor operators) for automatically operating the actuator shafts for these switches have become available. These systems must derive their power from DC batteries to allow the switch to be operated when AC electrical power is not available. Most of these systems are outfitted with a simple, microprocessor-based computer known in the industry as a Remote Terminal Unit (RTU) which allows the actuator/switch assembly to be operated remotely using radio, fiber, telephone or other commonly available data communication technologies.
Inside the motor operator various techniques have been developed for converting the electromotive force of the motor into the force necessary to operate the actuator shaft through a rotation of approximately 90 degrees (for the torsional operated actuator) or through a vertical motion of approximately one foot (for the reciprocating actuator). The most common methods have involved either an A.C./D.C. motor and gearbox assembly or a variety of hydraulic arrangements involving electric motor-driven pumps, pressure chambers, valves, etc.
One feature common to all of these motor operators is an operating requirement that the power distribution switch contacts be positioned and tensioned properly and that the closing and opening motion of the actuator be crisp and fast. This is necessary to ensure that when closed, the switch contacts provide maximum surface area for current flow and are firmly held in place to prevent vibration and arcing due to the alternating EMF. During opening and closing operations, arcing around the contacts will occur. Proper extinguishing of the arc requires that the switch mechanism move quickly.
Although the switch assemblies require fairly precise operation of the actuator shaft, the control systems for these motor operators have typically been fairly simple, consisting of electromechanical assemblies of actuating relays or motor contactors which are engaged to initiate an operation, and electromechanical limit switches on the output shaft that automatically stop the motor and perform any related functions (close valves, apply brakes, etc.) at or near the desired stopping point. For the vast majority of these control systems, feedback to the lineman, remote human operator or automated switching system has been fairly primitive, consisting of simple indications of battery status, switch position and other indications that can be developed from proximity sensors such as whether or not the cabinet door is open, the manual operating handle is present in the cabinet, or the operator and its actuator assembly are mechanically engaged.
These operators have performed reasonably well under ideal conditions at the time of initial commissioning. However, many problems arise as environmental factors cause changes to the operating conditions of the switch assemblies, operators and available energy in batteries. These problems can result in improper switch operation with potentially serious implications.
The most obvious problems result from one or more of the switch contacts becoming stuck to its mating surface due to a buildup of ice or due to welding (caused by high-current surges related to lightning discharge). This will generally leave the motor running until its circuit breaker or fuse trips. Unless the breaker automatically resets, the switch will require immediate, human intervention, and even with automatic resetting, further successful operation is unlikely. A further undesirable circumstance is that the mechanism is tensioned to open. In the case of a welded contact, the non-welded contacts are no longer firmly held in position. In the case of ice, melting of the ice before the problem is corrected could leave the switch partially open.
Another problem relates to variations in the force necessary to overcome the problem of stuck or sticky switch contacts and debris or ice buildup. The operator must be designed to apply the necessary force without applying so much force that the switching mechanism is readily damaged. Since the force applied to the switch depends on many factors, this is a difficult problem to solve with a simple, electromechanical design. In the event that excessive force is applied for any reason, it is desirable to be able to indicate this to the remote operator so that preventative maintenance or inspection can be performed.
Another problem results from uncontrollable variations in speed of operation. These variations are caused by variations in the voltage supplied from the battery due to state of charge, age, temperature, etc. They are also caused by variations in the condition of the switching mechanism including slack, corrosion, etc. In most cases, the limit switches used in existing operators must be set to trip prior to the desired stopping point to allow the operator to slow down and stop. This requires that the stopping distance be consistent with the initial setting of the limit switches. Changes in stopping distance based upon variation in speed at time of limit switch contact cannot be adequately taken into account in such an electromechanical arrangement.
Another problem with existing designs is the difficulty in creating indications for the remote operator when something otherwise obvious is wrong. For example, if the actuator is mechanically disengaged from the switch, the motor operator may appear to function normally but without moving the switch to the desired position. Although proximity sensors can be deployed to detect these conditions, these sensors must be placed in hostile, outdoor environments that are costly to properly instrument and unreliable in function.
Another problem relates to the complexity of adjusting the digital limit switches, and to their inherent unreliability. The mechanical adjustment of these switches is difficult to perform reliably, with only very subjective criteria for correct setting.
Another problem relates to the method of coupling the operator to the switch. This is typically performed with a slip fitting held in place with U-bolts. This allows the operator and switch to be mechanically connected without regard to limit switch settings. Without the slip fitting, typical operators would have to be somehow rotated to the correct position before the couplings were connected. These couplings are problematic and can slip during high-torque operation, causing the switch to go out of adjustment.
Another problem relates to the dangerous situation posed by a lineman operating the device from the front panel. Existing designs have no way to allow the operator to get out of the way before the switch operates.